Anti-causal vehicle suspension

ABSTRACT

A vehicle suspension and wheel damper using anti-causal filtering. Information from in front of a wheel or information about an operating condition of a vehicle is used to anti-causally determine a response of an active suspension associated with the wheel.

BACKGROUND

This specification relates to vehicle suspensions and vehicle wheeldampers. Vehicle suspensions attempt to eliminate or reduce verticaldisplacement of a sprung mass, typically including a passengercompartment, resulting from the vehicle encountering road disturbances.Wheel dampers attempt to eliminate or reduce “wheel hop” which is atendency for a vehicle wheel to lose contact with the road under somecircumstances.

SUMMARY

In one aspect, a method for operating a suspension system associatedwith a wheel of a vehicle includes identifying a road disturbance priorto the wheel's encountering the road disturbance; determining, prior tothe wheel's encountering the road disturbance, an estimated response ofthe suspension system to the road disturbance; and reversing withrespect to time the estimated response of the suspension system toprovide an anti-causal response. The method may further include scalingthe anti-causal response to provide a scaled anti-causal response. Thedetermining may determine the estimated response over a first period oftime and the method may include causing the controllable force source toapply the anti-causal response over a second period of time, shorterthan the first period of time. The wheel may be a rear wheel and theidentifying the road disturbance may include processing data from amotion sensor associated with a front wheel. The method may be operatedin a narrow frequency band to control wheel hop. The method may beoperated in a broad frequency band not including the narrow frequencyband to control the vertical displacement of a car body. The method mayfurther include filtering the anti-causal response with a nonlinearfilter. The nonlinear filter may be one of a deadband filter or aclipper. The anti-causal response may cause the suspension system toreduce vertical motion of an unsprung mass of the vehicle over a firstfrequency range and controls vertical motion of a sprung mass of thevehicle over a second frequency range embracing the first frequencyrange but not including the first frequency range.

In another aspect, a method for operating an active suspension systemassociated with the wheel of a vehicle includes identifying a roaddisturbance prior to the wheel's encountering the road disturbance;determining, prior to the wheel's encountering the road disturbance,what at least two states of the vehicle would be when the wheelencounters the road disturbance to provide estimated state values;determining, prior to the wheel's encountering the road disturbance, aforce to cause the at least two states of the vehicle to have theestimated state values to provide an anti-causally determined force; andcausing a controllable force source, associated with the activesuspension system, to apply the anti-causally determined force so thatthe values of the at least two states of the vehicle are the estimatedstate values when the wheel encounters the road disturbance. Thedetermining may include determining an estimated response of thesuspension system to the road disturbance, prior to the wheel'sencountering the road disturbance; reversing with respect to time theestimated response of the suspension system to provide a reversedanti-causal response; and causing a controllable force source to applythe reversed anti-causal response prior to the wheel's encountering theroad disturbance. The two state values may be the vertical position ofthe wheel and the vertical velocity of the wheel. The method may beoperated in a narrow frequency band to control wheel hop. The method maybe operated in a broad frequency band not including the narrow frequencyband to control the vertical displacement of a car body. The method mayfurther include filtering the anti-causal response with a nonlinearfilter. The nonlinear filter may be one of a deadband filter or aclipper. The anti-causally determined force may cause the suspensionsystem to reduce vertical motion of an unsprung mass of the vehicle overa first frequency range and to control vertical motion of a sprung massof the vehicle over a second frequency range embracing the firstfrequency range but not including the first frequency range.

In another aspect, a method for operating an active suspension for avehicle includes anti-causally calculating a force to be applied to aportion of the vehicle; calculating a predicted behavior of the portionof the vehicle resulting from the force being applied to the portion ofthe vehicle; comparing the predicted behavior with the actual behaviorof the vehicle to provide an error signal; determining a correctingforce to reduce the error signal; combining the correcting force withthe anti-causally calculated force to provide a combined force; andapplying the force to the vehicle portion. The portion of the vehiclemay be an unsprung mass of the vehicle. The method may be operated in afrequency band included a wheel hop resonance frequency to controlvertical motion of the unsprung mass. The portion of the vehicle may bethe sprung mass of the vehicle.

In another aspect, an apparatus includes a vehicle cabin; unsprungvehicle components, including a wheel; a suspension element, comprisinga sensor for identifying a road disturbance prior to the wheel'sencountering the road disturbance; logic for anti-causally determining aforce to reduce vertical displacement of a portion of the vehicleresulting from the wheel's encountering the road disturbance; and aforce source for exerting the anti-causally determined force prior tothe wheel's encountering the road disturbance. The logic foranti-causally determining the force may determine a force to reducevertical displacement of the unsprung mass over a first range offrequencies and may determine a force to reduce displacement of thesprung mass over a range of frequencies above the first range offrequencies and a range for frequencies below the first range offrequencies. The logic for anti-causally determining the force to reducevertical displacement of a portion may include logic for determining,prior to the wheel's encountering the road disturbance, an estimatedresponse of the suspension system to the road disturbance; and forreversing with respect to time the estimated response of the suspensionsystem to provide the anti-causally determined force. The logic foranti-causally determining the force to reduce vertical displacement of aportion may include logic for determining, prior to the wheel'sencountering the road disturbance, what at least two states of thevehicle would be when the wheel encounters the road disturbance toprovide estimated state values; and for determining, prior to thewheel's encountering the road disturbance, a force to cause the at leasttwo states of the vehicle to have the estimated state values to providean anti-causal force. The logic for anti-causally determining the forceto reduce vertical displacement of a portion includes logic forcalculating, from a model of a road surface and a model of the vehicle,a predicted behavior of the portion of the vehicle resulting from theforce being applied to the portion of the vehicle; for comparing thepredicted behavior with the actual behavior of the vehicle to provide anerror signal; for determining a correcting force to reduce the errorsignal; for combining the correcting force with the anti-causallycalculated force to provide a combined force; and for applying the forceto the vehicle.

Other features, objects, and advantages will become apparent from thefollowing detailed description, when read in connection with thefollowing drawing, in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a logical arrangement of a vehicle suspension element;

FIG. 2A is a logical arrangement of a vehicle suspension element;

FIG. 2B is a block diagram of the vehicle suspension element of FIG. 2A;

FIG. 3A is a logical arrangement of a vehicle suspension element;

FIG. 3B is a block diagram of the vehicle suspension element of FIG. 3A;

FIG. 4 is a block diagram of a feedback controller;

FIG. 5 is a diagram illustrating the operation of various types ofsuspension and wheel dampers as a function of frequency;

FIG. 6A is a logical arrangement of a front and a rear suspensionelement;

FIG. 6B is a block diagram of the front and rear suspension elements ofFIG. 6A;

FIG. 7 is a block diagram of an anti-causal filter;

FIG. 8 is a block diagram of a rear suspension controller;

FIGS. 9A-9D are plots of actuator force vs. time;

FIG. 10A is a logical arrangement of a front and a rear suspensionelement;

FIG. 10B is a block diagram of the front and rear suspension elements ofFIG. 10A;

FIG. 11A is a logical arrangement of a suspension element; and

FIG. 11B is a block diagram of the suspension element of FIG. 11A.

DETAILED DESCRIPTION

Though the elements of several views of the drawing may be shown anddescribed as discrete elements in a block diagram and may be referred toas “circuitry”, unless otherwise indicated, the elements may beimplemented as one of, or a combination of, analog circuitry, digitalcircuitry, or one or more microprocessors executing softwareinstructions. The software instructions may include digital signalprocessing (DSP) instructions. Operations may be performed by analogcircuitry or by a microprocessor executing software that performs themathematical or logical equivalent to the analog operation. Unlessotherwise indicated, signal lines may be implemented as discrete analogor digital signal lines, as a single discrete digital signal line withappropriate signal processing to process separate streams of signals, oras elements of a wireless communication system. Some of the processesmay be described in block diagrams. The activities that are performed ineach block may be performed by one element or by a plurality ofelements, and may be separated in time. This specification describes avehicle suspension system. For simplicity of explanation, some of thefigures and explanations show and describe a single wheel andsuspension. Actual implementations may have four or more wheels and acorresponding number of suspensions. Some of the figures andexplanations show a front wheel and suspension and a rear wheel andsuspension. Actual implementations may have two or more sets of frontand rear wheels and corresponding suspensions.

FIG. 1 shows a logical arrangement of a prior art single wheel vehiclesuspension system. The suspension 20 includes a spring 22 and a damper24. In operation, the spring 22 acts to oppose, and the damper 24 actsto damp, vertical motion of the sprung mass Ms 44 (that is, thecomponents of the vehicle that are supported by the spring 22, forexample including the passenger compartment). Control elements 36 and 38will be described below.

In the suspension of FIG. 1, the spring 22 exerts a force only if avertical mechanical stimulus (for example resulting from a roaddisturbance) is applied to the spring. The magnitude of the force isF_(spring)=−kx (where k is a constant representing the stiffness of thespring and x is the displacement of the spring). The magnitude of theforce exerted by the spring is determined solely by the displacement xand the current value of the spring constant k. A control element 36 maymodify the stiffness k of the spring, but it cannot cause the spring toexert a force absent any displacement, nor can it cause the spring toexert a force with a magnitude other than |kx|. Furthermore, the forceexerted by the spring can only be in a direction opposite to thedisplacement. The force cannot be applied before the displacementoccurs. Suspensions with these characteristics will be referred toherein as “passive”. Passive suspensions with a control element 36 whichcan vary the stiffness k may be referred to as “semi-active”, but arenot “active” suspensions, as the term is used herein.

The damper 24 exerts a force F_(damping)=−cv, where v is the relativevertical velocity between the unsprung mass and the sprung mass and c isa damping coefficient. A control element 38 may vary the value of thedamping coefficient c, but the damping force may never be of a magnitudeother than cv and must be in the opposite direction of v, as indicatedby the minus sign. Dampers with these characteristics will be referredto herein as “passive”. Dampers with a control element 38 that can varythe value of c will be referred to as “semi-active”, but are not“active” dampers, as the term is used herein.

Since the tire 28 has a compliance, the tire 28 and the unsprung mass 47(that is, the portion of the vehicle not supported by the spring 22, forexample, including the tire 28, the knuckle, brakes and all other partsthat move vertically with the wheel) can be modeled as an unsprung mass47 and a spring 32 representing the compliance of the tire. The spring32 and mass 47 are components of a resonant system that has a mechanicalresonance at a resonant frequency f_(res), typically around 12 Hz. Thetire is itself typically lightly damped, so if the tire is excited atfrequency f_(res), a significant amount of vertical motion of the wheelmay occur, which may cause the tire to lose contact with the road; thisis often referred to as “wheel hop”.

Wheel hop is undesirable because the partial or complete loss of contactbetween the tire and the road affects handling and braking. Therefore,various schemes have been developed, for example as described by U.S.Pat. No. 2,955,841 and U.S. Pat. No. 5,392,882 to damp wheel hop. Thedamping systems typically include a second damper and a damping mass(for example element 68 of U.S. Pat. No. 2,955,841 and element 40 ofU.S. Pat. No. 5,392,882). Damping masses are undesirable because theyadd to the number of mechanical components, the weight and the bulk ofthe vehicle. In the conventional suspension system of FIG. 1, the damper24 damps the wheel motion.

FIG. 2A shows a logical arrangement of a controllable suspension system140. The controllable suspension system includes an independentlycontrollable force source 42 which couples the sprung portion of thevehicle, including the passenger compartment, modeled as mass Ms 44 andunsprung mass modeled as mass Mu 47, which includes a wheel 46 (plus theknuckle, brakes, and all other parts that move vertically with thewheel), which engages the road or other surface on which the vehicle istravelling. Coupled to the force source 42 is control circuitry 48 whichcauses the force source 42 to generate a vertical force between thesprung mass 44 and the unsprung mass 47. By convention, positive forceis defined as force that urges the sprung mass and the unsprung massapart, and negative force is defined as force that urges the sprung massand the unsprung mass together. Unlike the spring 22 of FIG. 1, theforce source 42 can generate force in an arbitrary phase and magnitudebased on instructions from the control circuitry, and the applicationforce does not need to be in response to any direct stimulus or todisplacement, and can even occur before the wheel encounters a roaddisturbance. Suspension systems with these characteristics will bereferred to herein as “active”. The suspension system may have a sensor50 on or in the sprung mass. Sensor 50 is operationally coupled to thecontrollable force source 42 and detects vertical displacement. Thesprung mass 44 and the unsprung mass 47 may be mechanically coupled by apassive suspension element, such as a support spring 43 to support thestatic weight of the sprung mass. The support spring 43 may be in seriesor in parallel with the force source 42. The support spring 43 is notrelevant to the suspension system described in this specification, so itwill be omitted from future figures. An example of a controllablesuspension 140 can be found in U.S. Pat. No. 4,981,309 and U.S. Pat. No.7,195,250.

In some implementations, sensor 50 may not detect vertical displacementdirectly, but instead may detect a quantity, such as acceleration orvelocity, from which displacement can be derived. In someimplementations, vertical displacement may not be derived at all, butinstead the control circuitry may be designed to operate on the quantityfrom which vertical displacement can be derived. For example,accelerometers may be used, and the control circuitry may be designed tooperate on acceleration data directly rather than converting theacceleration data to vertical displacement data.

FIG. 2B represents the logical arrangement of suspension system 140 ofFIG. 2A, expressed as elements of a block diagram of a closed loopfeedback control system. Like numbers refer to the block diagramrepresentation of the corresponding elements of FIG. 2A. Roaddisturbances cause a force to be applied to unsprung mass 47. At lowerfrequencies, the force applied to the unsprung mass 47 is transmitted tothe sprung mass 44. The sensor 50 detects vertical displacement of thesprung mass 44 and provides a signal representative of verticaldisplacement to control circuitry 48, which determines an error signalthat represents the difference between the desired vertical displacement(typically zero) and the actual vertical displacement. The magnitude,phase, and direction of a force to drive the error signal toward zeroare determined by the control circuitry. The control circuitry 48directs the force source 42 to apply the force determined by the controlcircuitry 48 between the sprung mass 44 and the unsprung mass 47. Byconvention, positive force is defined as urging the sprung mass and theunsprung mass apart, and negative force is defined as urging the sprungmass and the unsprung mass together. The system of FIGS. 2A and 2B actsto reduce the vertical displacement of the sprung mass. However, thesystem of FIGS. 2A and 2B does not control the vertical displacement ofthe unsprung mass, so the system of FIGS. 2A and 2B does not necessarilyact to reduce wheel hop. Schemes have been developed, for example asdescribed U.S. Pat. No. 4,991,698 and U.S. Pat. No. 6,364,078, for usewith suspensions such as shown in FIGS. 2A and 2B to damp wheel hopseparately from the controllable suspension system 140. U.S. Pat. No.4,991,698 and U.S. Pat. No. 6,364,078 both include damping masses, andboth are “passive” systems (as defined above) that are in addition to,and supplement, the conventional passive or semi-active suspensionsystem of FIG. 1.

A suspension system 240 for damping wheel hop as well as reducingvertical displacement of the sprung mass is shown in FIGS. 3A and 3B.For simplicity of explanation, the suspension system 240 is described inthe form a logical arrangement in FIG. 3A and in the form of a blockdiagram of a feedback loop in FIG. 3B. Logical equivalents of thesuspension system can be implemented by many different combinations ofphysical elements and microprocessors executing different combinationsand sequences of instructions and calculations.

In the logical arrangement of FIG. 3A, the controllable suspensionsystem 240 includes an independently controllable force source 42 whichcouples the sprung portion of the vehicle, including the passengercompartment, modeled as mass Ms 44 and unsprung mass modeled as mass Mu47, which includes a tire 46 (plus the knuckle, brakes, and all otherparts that move vertically with the wheel), which engages the road orother surface on which the vehicle is travelling. Coupled to the forcesource 42 is control circuitry 248 which causes the force source 42 togenerate a vertical force between the sprung mass 44 and the unsprungmass 47. By convention, positive force is defined as force that urgesthe sprung mass and the unsprung mass apart, and negative force isdefined as force that urges the sprung mass and the unsprung masstogether. Similar to the force source of FIGS. 2A and 2B, the forcesource 42 of FIGS. 3A and 3B can generate force in an arbitrary phaseand magnitude based on instructions from the control circuitry 248, andthe force does not need to be in response to any direct stimulus or todisplacement. The suspension system may have a sensor 50 on or in thesprung mass and a sensor 54 in or on a non-rotating part of the unsprungmass. Sensors 50 and 54 are operationally coupled to the controlcircuitry 248 and detect vertical displacement (or some quantity such asvertical acceleration or vertical velocity from which verticaldisplacement can be derived) of the sprung mass and the unsprung mass,respectively. As with the suspension system of FIGS. 2A and 2B, thesprung mass 44 and the unsprung mass 47 may be mechanically coupled by apassive suspension element, such as a support spring which is notrelevant to the suspension system described in this specification, so itis omitted.

FIG. 3B represents the suspension system 240 of FIG. 3A, expressed aselements of a block diagram of a closed loop feedback control system.Like numbers refer to the block diagram representations of thecorresponding element of FIG. 3A. Road disturbances cause a force to beapplied to unsprung mass 47. At lower frequencies, the force istransmitted to sprung mass 44 and at frequencies near f_(res), the forcefrom the road disturbance may cause wheel hop. Sensor 50 detectsvertical displacement of the sprung mass 44 and sensor 54 detectsvertical displacement (wheel hop) of the unsprung mass 47. Based oninput from the sensors, an error signal is provided to a feedback loopcontroller 57. In some embodiments, for example as shown in FIG. 3B, theerror signal provided to the controller 57 includes output of theunsprung mass sensor 54 at frequencies near f_(res) and does not includethe vertical displacement of the unsprung mass sensor 54 at otherfrequencies. The feedback loop controller 57 determines a force to beapplied by the force source 42 between the sprung mass 44 and theunsprung mass 47 to reduce the vertical displacement of the unsprungmass at frequencies near f_(res) and to reduce the vertical displacementof only the sprung mass at other frequencies. By convention, positiveforce is defined as urging the sprung mass and the unsprung mass apart,and negative force is defined as urging the sprung mass and the unsprungmass together.

FIG. 4 shows one implementation of the feedback loop controller 57. Thefeedback loop controller includes a feedback loop controller 571 and anon-linear processor 572.

In operation, a force determiner 571 determines a force to be applied byforce source 42. Before the force command is sent to the force source42, the force command may be processed by a non-linear processor 572.For example, a deadband filter might be used so that small dampingforces (the application of which consumes energy but does notsubstantially improve ride comfort) are zeroed out. Alternatively to, orin combination with, a deadband filter, a clipper might be used so thatextremely large forces determined by the feedback loop controller arelimited to a maximum value. It may also be desirable to further processthe output of the non-linear filter with a smoothing filter. Forexample, the output of a clipper might be sent to a low-pass filter thatremoves high frequency content so that harshness is not injected intothe vehicle by the force source 42.

FIG. 5 illustrates various combinations of active and passivesuspensions and passive dampers, and shows which masses (sprung orunsprung) are controlled by which suspension elements at variousfrequency ranges. Diagram 60 shows the operation of a system forexamples as shown in FIG. 1, including a passive (as defined above)suspension and a passive (as defined above) wheel damper. The passivewheel damper damps vertical displacement of the unsprung mass throughoutits range of operation, typically including f_(res), in this example, 12Hz. The passive suspension controls vertical displacement of the sprungmass throughout its range of operation. The range of operation of thepassive suspension may include the range of operation of the passivewheel damper, as indicated by the dashed line. Diagram 62 shows theoperation of a system including an active (as defined above) suspensionand a passive damper (for example as described U.S. Pat. No. 4,991,698and U.S. Pat. No. 6,364,078 damper, which include mass dampers). Similarto diagram 60, in diagram 62, the passive wheel damper damps verticaldisplacement of the unsprung mass throughout its range of operation,typically including f_(res), in this example, 12 Hz. The activesuspension controls vertical displacement of the sprung mass throughoutits range of operation. The range of operation of the passive suspensionmay include the range of operation of the passive wheel damper, asindicated by the dashed line. Diagram 64 represents the operation of thesystem of FIGS. 3A and 3B, which do not include a passive wheel damper.The active suspension/damper controls vertical displacement of theunsprung mass through an unsprung mass damping band of frequenciesincluding f_(res), in this example, 12 Hz. In frequency bands above andbelow the unsprung mass damping band of frequencies, the activesuspension/damper controls vertical displacement of the sprung mass.

A suspension system according to FIGS. 3A and 3B may permit verticaldisplacement of the sprung mass in the unsprung mass damping band offrequencies. Since the vertical displacement of the sprung mass might beperceived by the occupants of the vehicle cabin, it is desirable for theunsprung mass damping band of frequencies to be as narrow as possible,as illustrated by diagram 66, which will be described below.

In one implementation, the force source 42 is an electromagneticactuator as described in U.S. Pat. No. 7,963,529. The controller 248 isa microprocessor processing software instructions. The sensors 50 and 54may be, for example, displacement sensors as described in U.S. Pat. No.5,574,445 or, as previously stated, may be accelerometers or sensor ofsome other quantity from which vertical displacement can be derived.Sensor 54 may be mounted in a non-rotating part of the wheel.

The suspension system of FIGS. 6A and 6B shows front and rear suspensionsystems, 240F and 240R, respectively. Each of the front and rearsuspension systems includes the same elements as the suspension systemof FIGS. 3A and 3B, which an additional element 70R that will beexplained below. FIG. 6B represents the suspension system of FIG. 6A,expressed as elements of a block diagram of a closed loop feedbackcontrol system. Like numbers are the block diagram representations ofthe corresponding element of FIG. 6A. The “R” suffix refers to elementsof the rear suspension system and the “F” suffix refers to elements ofthe front suspension system. In the suspension system of FIGS. 6A and6B, information (for example the pattern of disturbances of the road)from the front suspension 240F is provided to the rear suspension system240R, as indicated by line 51. The front suspension system 240F and therear suspension system 240R may be operationally coupled by line 51through an anti-causal (sometimes referred to as acausal) filter 70R.Alternatively, the anti-causal filter may logically be considered a partof the rear control circuitry 248R. An anti-causal filter is a filterwith an impulse response that has content before t=0 and will bediscussed in more detail below.

“Front” and “rear”, as used below, are defined as consistent with thedirection of travel such that the front wheels are the set of wheelsthat lead and encounter road disturbances before the rear wheels. Sincethe front and rear wheels on each side of a vehicle typically encounterthe same road disturbances at different times, the informationcommunicated from the front suspension system to the rear suspensionsystem indicated by line 51 provides a preview (in distance and time) ofthe road pattern of road disturbances. The amount of distance previewdepends on the wheelbase of the vehicle and the speed at which thevehicle is traveling. For example, a small vehicle with a 2.5 mwheelbase traveling at a high speed of 35 m/sec has about 0.07 of secpreview and a large vehicle with a 3 m wheelbase traveling at a slowspeed of 10 m/sec has about 0.3 sec of preview. In this interval, therear suspension system control circuitry 248R can prepare the rearsuspension system so that it can perform better (in terms of one or moreof the comfort of the vehicle passengers, energy consumption, andcontrol of wheel hop) than if the preview information were notavailable.

In operation, the front suspension system operates in the mannerdescribed above in the discussion of FIGS. 3A and 3B. Road informationis provided to the rear anti-causal filter 70R as indicated by line 51.The rear anti-causal filter 70R calculates an open loop force patternappropriate to the road information in a manner that will be discussedbelow in the discussion of FIG. 7. The open loop force pattern may thenbe provided to the rear force source 42R. In theory, the rear suspensionsystem of FIGS. 6A and 6B can be operated as a completely anti-causalopen loop system; that is, the rear force source 42R applies an openloop force pattern determined by rear anti-causal filter 70R only. Therear control circuitry 248R may merely pass through the open loop forceand may perform no additional processing, so that the verticaldisplacement data from rear sprung mass sensor 50R and rear unsprungmass sensor 54R are unnecessary, as indicated by the dashed lines.However, it may be advisable for the rear controller 248R to adjust theopen loop force using vertical displacement data from rear sprung masssensor 50R and rear unsprung mass sensor 54R to correct for imprecisionin the models and assumptions used by the anti-causal filter 70R and byother factors, for example if the rear wheel tracks slightly differentlythan the front wheel.

In an alternate arrangement, the signal from front sensor 54F could betransmitted directly to the rear controller 57R, and the rear controllercould determine a force based on the information from front sensor 54F,as shown in dashed lines if FIG. 6B.

FIG. 7 shows a block diagram representation of an anti-causal filtersuitable for use as element 70R. Block 71 represents the optionalapplication of delay, so that the open loop force determined by theanti-causal filter 70R is applied at the correct time. Block 72represents a bandpass filtering of the pattern of vertical disturbancesof the road, which may be a causal filtering. Block 74 represents ananti-causal filtering. The filtering block 74 implements a time reversedbandpass filtering operation that is possible because information abouta road disturbance to be filtered is available to the filter before therear wheel encounters the disturbance. Block 76 represents a calculatedestimation of the force to be applied by rear force source 42R. Block 76can be determined using a full ¼-vehicle model or a measured planttransfer function for the vehicle. In one implementation, the force iscalculated according to:

${\Delta\; z_{tire}} = {{{H_{road}z_{road}} + {H_{force}f_{actuator}}} = {\left. 0\Rightarrow f_{actuator} \right. = {{- \frac{H_{road}}{H_{force}}}z_{road}}}}$where Δz_(tire) is the deformation of the tire, where a positive numberdenotes expansion and a negative number denotes compression; H_(road) isthe frequency response function from road displacement to tireexpansion; z_(road) is the vertical position, relative to an inertialreference frame, of the road under the contact patch of the tire;H_(force) is the frequency response function from force produced in theactuator to tire expansion; and f_(actuator) is the actuator force. Thisequation calculates the force it would take to keep the tire from everexpanding or compressing. The sprung mass 44 has little effect at thewheel hop frequency f_(res), so it may be neglected, and we can writethe above equations for a ¼ vehicle model to calculate the actuatorforce necessary to prevent the tire from deforming or to prevent thewheel from moving vertically. In the two cases, we get:

${\Delta\; z_{tire}} = {{{{- \frac{m_{u}s^{2}}{{m_{u}s^{2}} + {b_{t}s} + k_{t}}}z_{road}} - {\frac{1}{{{+ m_{u}}s^{2}} + {b_{t}s} + k_{t}}f_{actuator}}} = {\left. 0\Rightarrow f_{actuator} \right. = {m_{u}s^{2}z_{road}}}}$$z_{wheel} = {{{\frac{{b_{t}s} + k_{t}}{{m_{u}s^{2}} + {b_{t}s} + k_{t}}z_{road}} - {\frac{1}{{{+ m_{u}}s^{2}} + {b_{t}s} + k_{t}}f_{actuator}}} = {\left. 0\Rightarrow f_{actuator} \right. = {\left( {{b_{t}s} + k_{t}} \right)z_{road}}}}$where m_(u) is the unsprung mass; s is complex frequency vector; b_(t)is the damping in the tire; and k_(t) is the tire vertical stiffness.Referring again to FIGS. 6A and 6B, the calculated forces could then beapplied by rear force source 42R. The calculated forces can be the soleinstructions to the rear force source 42R so that the rear suspensionsystem is operated in an open loop manner. The additional output ofblock 74 and the output of block 76 will be discussed later. Some of theoperations of anti-causal filter 70R may be performed in a differentorder than shown. For example, the operation of block 71 may be appliedin any order relative to the other operations; the operations of blocks72 and 74 may be performed as shown, or in reverse order.

Referring again to FIG. 7, the operations of blocks 72, 74, and 76 canbe performed by a finite impulse response (FIR) filter 77. In oneimplementation, the FIR filter bandpasses z_(road) forward and backwardwith a second order anti-notch filter; calculates the impulse responsein the actuator force of a damped system; reverses with respect to timethe response (as will be discussed later); and may scale the reversedimpulse response to build up a portion, for example half, of the totalenergy of the suspension system (that is, the energy injected into thesuspension system and the energy dissipated by the suspension system),so that a part of the energy is injected into the suspension systembefore the rear wheel encounters the road disturbance and the remainderis dissipated after the rear wheel encounters the road disturbance. Thebuild up and dissipating of total energy of the suspension system willbe discussed below in the discussion of FIGS. 9A-9D). In otherimplementations, other filter topologies, for example IIR filters, couldbe used.

The delay (block 71 of FIG. 7) can be thought of either in terms of time(the time between when sensor 54F detects the road disturbance and whenthe force is to be applied, which can be before the rear wheelencounters the road disturbance) or in terms of distance (related to thewheel base).

Generally, the use of more complex filters and the capability ofexerting the force more in advance of the wheel encountering thedisturbance results in a combination of less vertical displacementexperienced by the occupants of the vehicle, more efficient energyusage, and lower peak force requirements for the force source 42.

As stated in the discussion of FIGS. 3A and 3B, it is desirable for theband of frequencies in which the active suspension controls displacementof the unsprung mass and does not necessarily control the verticaldisplacement of the sprung mass to be as narrow as possible. Forexample, in one implementation, filters 72 and 74 are a fourth orderhighpass filter with a break frequency at the tire hop frequency (forexample 12 Hz) multiplied by a second order lowpass filter with a breakfrequency at the tire hop frequency. The resulting band pass filter isadjusted in gain to have a unity gain at the tire hop frequency andpassed forward and backward, resulting in a net magnitude effect of aneighth order roll off above and below the tire hop frequency, withoutany phase lag. The use of information from the front wheel permits therear wheel suspension to use a relatively high order (in this exampleeighth order) filter, which results in a narrower bandwidth of theunsprung mass damping frequency band, which results in less verticaldisplacement being perceived by the occupants of the vehicle.

As stated previously, in theory the rear controller 248R can operate asan open loop system, and it is theoretically not necessary for the rearcontroller to perform any additional calculation to control wheel hop.However, in actual implementations, it may be advisable for the rearcontroller 248R to improve the performance of the rear suspension systemby correcting for calculation errors. FIG. 8 shows some elements of asuitable rear controller 248R and some other elements of the suspensionsystem of FIG. 6B. The rear controller 248R includes a delayed roadmodel 502 operationally coupled to a plant model 504. The plant model504 may be simple, for example, modeled as a spring representing thetire, and a mass, representing the unsprung mass. The plant model couldalso be more complex, for example, also including the sprung mass 44 ofFIGS. 2A, 3A, and 6A, the support spring 43 of FIG. 2A, a damper betweenthe sprung mass 44 and the unsprung mass 47, and the force source 42.The plant model 504 is coupled to force calculation circuitry 506 and tothe output of unsprung mass sensor 54R through summer 508. The forcecalculation circuitry 506 is coupled to the force source 42 throughsummer 510.

In operation, road model 502 receives input from block 74 of FIG. 7 andapplies a an additional time delay to account for the full delay betweenthe front and rear wheels impacting the road disturbance. The plantmodel 504 receives as input the force from force calculation block 76and the road estimate from delayed road model 502 and predicts theeffect of the force from block 76 on an unsprung mass with thecharacteristics stored in block 504 of a vehicle traveling on theestimated road from road model block 502. Summer 508 differentiallycombines the actual behavior (the vertical displacement) of the unsprungmass measured by unsprung mass sensor 54 with the predicted behavior(the vertical displacement) from block 504 so that the output of summer508 is an error signal representing the difference between the predictedbehavior and the actual behavior of the unsprung mass. The differencemay be attributable to imprecision of one or more of the road model, theplant model, or the calculated force or other factors, for example therear wheel tracking slightly differently than the front wheel. The forcecalculation block 506 then calculates the force necessary to correct forthe error signal from summer 508. The calculated force from block 508 isthen summed at block 510 with the force from block 76 and the summedforces are then applied by rear force source 42R.

FIGS. 9A-9D illustrate the anti-causal operation of the forcecalculation block 76 (or force calculation block with error correctingblocks shown in FIG. 8). In FIGS. 9A-9D, the vertical axis is forceapplied by the force source 42 (of previous figures), with positivevalues representing force urging the sprung mass and the unsprung massapart, and negative values representing the force urging the spring massand the unsprung mass together. The horizontal axis represents time,with t=0 representing the time at which the rear wheel encounters theroad disturbance; negative time values represent the application offorce before the rear wheel encounters the road disturbance. Theapplication of force, particularly at peak force points 84A and 86A, maybe perceived by the occupants of the vehicle cabin. FIG. 9A shows apurely causal damping command, where all force is applied after and inresponse to the road disturbance. The force pattern of FIG. 9A might bethe force pattern of the front wheel, or of the rear wheel in a systemin which information from the front wheel is not used by the rear wheelcontrol circuitry. FIG. 9B shows a theoretical use of force calculationblock 76. In. FIG. 9B, the actuator force causes system states, forexample, the position and velocity of the unsprung mass 47 and thesprung mass 44 to be the opposite at time t=0 of FIG. 9A. One way forcausing the system states to be the opposite as at time t=0 of FIG. 9Ais to reverse in time the response of curve 82A of FIG. 9A and apply thereversed response to the system so that at time t=0, the verticalvelocity (indicative of the kinetic energy) and the vertical position(indicative of the potential energy) of the unsprung mass are the sameat time t=0 of curve 82B of FIG. 9B as at time t=0 of curve 82A of FIG.9A. When the wheel encounters the impulse at time t=0, the energyimparted to the system by the impulse matches the energy removed fromthe system by the operation of the actuator and there is no resultantdisplacement in the system after the impulse. The operation of ananti-causal filter as shown in FIG. 9B, is illustrative theoretically,but would be of limited practical use. The occupants of the vehiclecabin would experience the same force profile as in FIG. 9A (andtherefore the same level of discomfort), except that the force profilewould be reversed and would occur before the vehicle encounters thedisturbance.

Curve 82C of FIG. 9C shows another example of the operation of forcecalculation block 76. In the example of FIG. 9C, the impulse responsethat would occur in the absence of the anti-causal filtering is reversedin time and applied with a scaled amplitude (for example so that halfthe energy imparted by the impulse is removed prior to time t=0). Theresult is that (unlike curve 82B of FIG. 9B) there is a responsesubsequent to time t=0, but the peak force applied by the actuator issignificantly less than in FIG. 9A or 9B, and the force is applied overa longer period of time.

Curve 82D of FIG. 9D is similar to curve 82C of FIG. 9C, except thescaled force is applied over a shorter period of time prior toencountering the disturbance, in this case about 0.2 seconds. Reducingthe period of time over which the force is applied may be desirable ornecessary if the time available for applying the force is limitedbecause the vehicle is traveling at a high rate of speed which decreasesthe time interval between the front wheel's encountering of thedisturbance and the rear wheel's encountering of the same disturbance.If the scaled force is applied over a shorter period of time, the peakforce required may be more than if the scaled force is applied over alonger time period. For example for the case of FIG. 9D,

${{\hat{F}}_{pre} = {F_{pre}\frac{\max\left( F_{post} \right)}{{\max\left( F_{post} \right)} + {\max\left( F_{pre} \right)}}}},$where {circumflex over (F)}_(pre) is the scaled pre-impact force,F_(pre) is the unscaled pre-impact force, and F_(post) is the postimpact force.

The application of force by the force source 42 according to curves 82Cand 82D of FIGS. 9C and 9D, respectively, is advantageous because thepeak energy usage is a lower magnitude, so that the peak force that theforce source 42 must be capable of applying can be lower, and themagnitude of the vertical displacement of the passenger compartment canbe less so that the vertical displacement is less perceptible (or evenimperceptible) by the occupants of the passenger compartment.

In addition to the energy related method of developing the force,calculation block 76 can use other techniques to develop the force. Forexample, the force pattern may be determined using finite time horizonlinear-quadratic regulator (LQR) techniques.

The techniques described in previous figures can be applied in otherways. For example, the information about road disturbances that thewheel has not yet encountered may be obtained from sources other than asensor associated with the front wheel and the concepts appliedpreviously to the rear wheel can also be applied to the front wheel. Forexample, the suspension system of FIGS. 10A and 10B includes all theelements of FIGS. 6A and 6B. The front control circuitry 248F or therear control circuitry 248R or both may have the same components as thelike numbered elements of FIGS. 3B and 6B. Some components of FIGS. 3Band 6B are not needed for the explanation of FIG. 10B and are not shownin this view. In addition to the components of FIG. 6B, the frontsuspension system 240F includes a front anti-causal filter 70F, and thesystem further includes a road disturbance detector 80, for example anoptical device located in the front bumper or a device that makesphysical contact with the road. The road disturbance detector detectsroad disturbances 30, 32. The road disturbance detector 80 provides theinformation to the front anti-causal filter 70F, which operates in themanner described above in the discussion of rear anti-causal filter 70R.There may be other methods for obtaining information about vertical roaddisturbances prior to a wheel encountering the disturbance. In anotherexample, instead of road disturbance detector 80, the suspension systemis provided with a road disturbance according to U.S. Pat. No. 7,195,250of sufficient granularity (for example, low pass filtered at a frequencygreater than f_(res)).

FIGS. 11A and 11B show another active suspension system. The suspensionsystems of FIGS. 11A and 11B include the elements of the suspensionsystems of FIGS. 3A and 3B. Elements of FIGS. 11A and 11B include thefeatures and functions of the similarly named elements of the suspensionsystems of FIGS. 3A and 3B. In addition to the elements of FIGS. 3A and3B, control circuitry 248R is operationally coupled to receiveinformation from the high speed vehicle bus 90. Information on the highspeed vehicle bus 90 includes information important to the operation ofthe vehicle. The controller 248R can use information from the high speeddata bus 90 to activate, deactivate, or alter the operation of the rearsuspension system. For example, traction is more important at some times(for example when the vehicle is accelerating, decelerating, or turning)that at other times (for example when the vehicle is traveling in astraight line at a constant speed), so when a situation in whichtraction is less important is detected, the elements relating to controlof the unsprung mass can be temporarily disabled (as indicated by thedashed lines) and vertical displacement of the sprung mass only can becontrolled over a full bandwidth. In another example, if the road issmooth, the unsprung mass control circuitry can reduce the gain of thecontrol circuit.

Numerous uses of and departures from the specific apparatus andtechniques disclosed herein may be made without departing from theinventive concepts. Consequently, the invention is to be construed asembracing each and every novel feature and novel combination of featuresdisclosed herein and limited only by the spirit and scope of theappended claims.

What is claimed is:
 1. A method for operating a suspension systemassociated with a wheel of a vehicle, comprising: identifying a roaddisturbance prior to the wheel's encountering the road disturbance;determining, prior to the wheel's encountering the road disturbance, anestimated response of the suspension system to the road disturbance;reversing with respect to time the estimated response of the suspensionsystem to provide an anti-causal response of the suspension system,determining a force for outputting by a controllable force sourceassociated with the suspension system, where the determining is based onthe time reversed anti-causal response of the suspension system, andoutputting the determined force by the controllable force source toreduce vertical motion of the vehicle wheel.
 2. The method of claim 1,further comprising scaling the anti-causal response to provide a scaledanti-causal response.
 3. The vehicle suspension of claim 1, wherein thedetermining an estimated response determines the estimated response overa first period of time and further comprises causing the controllableforce source to apply the determined force over a second period of time,shorter than the first period of time.
 4. The method of claim 1, whereinthe wheel is a rear wheel, wherein the identifying the road disturbancecomprises processing data from a motion sensor associated with a frontwheel.
 5. The method of claim 1, wherein the method is operated in anarrow frequency band to control wheel hop.
 6. The method of claim 5,wherein the method is operated in a broad frequency band not includingthe narrow frequency band to control the vertical displacement of a carbody.
 7. The method of claim 1, further comprising filtering theanti-causal response with a nonlinear filter.
 8. The method of claim 7,wherein the nonlinear filter is one of a deadband filter or a clipper.9. A method for operating an active suspension system associated withthe wheel of a vehicle, comprising: identifying a road disturbance priorto the wheel's encountering the road disturbance; determining, prior tothe wheel's encountering the road disturbance, what at least two statesof the vehicle would be when the wheel encounters the road disturbanceto provide estimated state values; determining, prior to the wheel'sencountering the road disturbance, a force to cause the at least twostates of the vehicle to have the estimated state values to provide ananti-causally determined force; and causing a controllable force source,associated with the active suspension system, to apply the anti-causallydetermined force so that the values of the at least two states of thevehicle are the estimated state values when the wheel encounters theroad disturbance.
 10. The method of claim 9, wherein the determiningcomprises determining an estimated response of the suspension system tothe road disturbance, prior to the wheel's encountering the roaddisturbance; reversing with respect to time the estimated response ofthe suspension system to provide a reversed anti-causal response; andcausing a controllable force source to apply the reversed anti-causalresponse prior to the wheel's encountering the road disturbance.
 11. Themethod of claim 9, wherein the two state values are the verticalposition of the wheel and the vertical velocity of the wheel.
 12. Themethod of claim 9, wherein the method is operated in a narrow frequencyband to control wheel hop.
 13. The method of claim 12, wherein themethod is operated in a broad frequency band not including the narrowfrequency band to control the vertical displacement of a car body. 14.The method of claim 9, further comprising filtering the anti-causalresponse with a nonlinear filter.
 15. The method of claim 14, whereinthe nonlinear filter is one of a deadband filter or a clipper.
 16. Themethod of claim 9, wherein the anti-causally determined force causes thesuspension system to reduce vertical motion of an unsprung mass of thevehicle over a first frequency range and to control vertical motion of asprung mass of the vehicle over a second frequency range embracing thefirst frequency range but not including the first frequency range.
 17. Amethod for operating an active suspension for a vehicle, comprising:anti-causally calculating a force to be applied to a portion of thevehicle; calculating a predicted behavior of the portion of the vehicleresulting from the force being applied to the portion of the vehicle;comparing the predicted behavior with the actual behavior of the vehicleto provide an error signal; determining a correcting force to reduce theerror signal; combining the correcting force with the anti-causallycalculated force to provide a combined force; and applying the force tothe vehicle portion.
 18. The method of claim 17, wherein the portion ofthe vehicle is an unsprung mass of the vehicle.
 19. The method of claim18, wherein the method is operated in a frequency band included a wheelhop resonance frequency to control vertical motion of the unsprung mass.20. The method of claim 17, wherein the portion of the vehicle is thesprung mass of the vehicle.